The target creature card is chosen as the triggered ability is put on the stack. Gaining life after that point won't let you choose a target with a higher mana value.

Celestine's triggered ability looks at the total amount of life you gained to determine X, regardless of whether or not you also lost any life. For example, if you gained 3 life this turn, X is 3, even if you also lost some amount of life in the same turn.